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Description
Cu($tn$)Cl$_2$ ($tn$ = C$_3$H$_{10}$N$_2$) represents a quasi-two-dimensional ($2$D) quantum magnet which preserves $2$D features far below the phase transition to the ordered state at $0.55$ K. The coexistence of $2$D and long-range magnetic correlations can be ascribed to the incommensurate modulated crystal structure. The modulation leads to the formation of extremely thin regions with a thickness running from one to four unit cells which differ in the orientation of $tn$ ligands. Since previous ab initio studies revealed high sensitivity of exchange interactions on the $tn$ positions, large variability of exchange couplings in such confined $2$D geometries can preserve the $2$D correlations. The inclusion of the two strongest nearest-neighbor exchange couplings from those provided by the ab initio studies leads to formation of effective rectangular lattice. While excellent agreement between specific heat data and the theory was achieved, significant deviations appear in the description of magnetic susceptibility and magnetic phase diagram, where additional phase in the vicinity of a critical region was observed [1]. Since the phase appears in the fields perpendicular to the direction of the modulation vector, its dependence on the field orientation may be caused by the spatial and spin anisotropies. The latter can be estimated from the electron paramagnetic resonance (EPR) spectra.
Present work is devoted to the X-band single crystal studies of angular and temperature dependence of EPR spectra providing information about the $g$-factors and linewidth $\Delta B$. The analysis of temperature dependence of linewidth along the $a$ and $b$ axis enabled the estimation of spin anisotropies. In the field parallel to $a$ axis which is perpendicular to modulation vector, the linewidth is determined by the contribution of antisymmetric Dzyaloshinskii-Moriya (DM) interaction, symmetric spin anisotropies of dipolar origin $K^{dip}$ and exchange anisotropy $K^{ea}$. The absence of additional phase in the magnetic phase diagram along the $b$ axis suggests that the DM contribution along the $b$ axis could be neglected and a major part of linewidth can be ascribed mainly to $K^{dip}$ and $K^{ea}$ anisotropies, which are always present in real materials. On the other hand, occurrence of DM interaction depends on the crystal symmetry. The possibility of EPR studies at higher frequencies is discussed for other characterization of spin anisotropies in Cu($tn$)Cl$_2$.
Acknowledgements
The financial support of projects VEGA 1/0132/22, APVV-18-0197 and APVV-22-0172 is acknowledged.
References
[1] R. Tarasenko et al., “Extraordinary two-dimensionality in the S = 1/2 spatially anisotropic triangular quantum magnet Cu(1,3-diaminopropane)Cl2 with modulated structure,” Physical Review B, vol. 108, no. 21. American Physical Society (APS), Dec. 28, 2023. https://doi.org/10.1103/physrevb.108.214432
